Many elastomers are tacky or exhibit cold flow in their green or uncured state. As a consequence, these materials cannot be transported in bulk as free flowing pellets but must be shipped in bales. This practice requires that the ultimate elastomer processor must be equipped to cut up or mill the bales. The necessary equipment is generally large scale, expensive equipment. Additionally, the bales cannot be readily preblended with other materials. The necessity for baling results in high handling and shipping costs. In order to facilitate handling and processing of elastomers, it has been considered desirable to produce elastomer pellets. Generally, however, elastomer pellets exhibit "blocking" or cold flow characteristics which result in solidification into a solid mass after a short storage time, especially at warm temperatures.
Numerous attempts have been made to formulate elastomeric pellets which will remain free flowing until they are to be processed. Dusting the elastomeric pellets with inorganic materials, e.g., clay, talc, etc., has been found to extend the time over which the pellets are free flowing. Improved results have been achieved by dusting a coating with selected organic materials such as hydrocarbon waxes (British Patent No. 901,664) or powdered polyethylenes and polypropylenes (British Patent No. 928,120). However, because of the discontinuity of the dust coat, the coated pellets eventually flow together to form a solid mass.
By blending the elastomer with a crystalline type polymer such as polyethylene, polypropylene or copolymers of ethylene and propylene, it has been possible to produce free flowing elastomer containing pellets. However, the elastomer content of the pellet must be less than about 65%. The product is, of course, not suitable for use in all elastomer applications.
Another coating approach to the problem has been the coating of elastomer pellets with emulsions containing a tack free coating material. Coating is accomplished either by dipping pellets into the emulsion or spraying the emulsion onto the pellets. In either case the emulsion coating must be dried, and where the emulsion contains a solvent the solvent must be recovered. Drying and solvent recovery requirements result in increased costs.
Melt-coating methods for producing free-flowing elastomer pellets have also been suggested. According to U.S. Pat. No. 3,669,772 to Bishop, coating can be accomplished by using a die, similar to wire coating die, into which a strand of rubber to be coated is fed simultaneous with melt coating material. A continuous melt coated strand of rubber issues from the coextrusion die outlet, is cooled in a liquid cooling bath, and is subsequently pelletized. This melt-coating method not only adds significantly to rubber manufacturing costs, but has limitations from the standpoint of efficiently producing large quantities of coated pellets.
Pellets of rubber have been coated with various coating materials by heating the rubber pellet to a temperature which is higher than the melting point of the coating material, and then contacting the heated pellet with the coating material which is preferably in the form of a fine powder. The heated pellet fluxes the coating material on the surface of the pellet to form a substantially continuous coating. The hot coated pellet is then cooled.
A study of bicomponent mixtures has shown that upon extrusion of the mixtures, stratification will occur. See Soulborn, J. H. and Ballman, R. L.; "Stratified Bicomponent Flow of Polymer Melts in a Tube", Applied Polymer Science, No. 20, 175-189 (1973). The authors attribute stratification to differences in the melt viscosity of the components.
What is required by the art is a commercially viable method of producing a free flowing rubber pellet which contains a major portion of rubber. To be commercially viable, the process for producing the free flowing pellets must be readily adapted to conventional rubber pelletizing techniques.